In a conventional EL display device, such as a TFT Organic Light Emitting Diode (TFT-OLED), one of the electrodes of the EL element contacts either the source or the drain electrode of the associated TFT device through a contact via hole in the underlying passivation layer. For example, schematically illustrated in FIG. 1 is a cross-sectional view of a conventional TFT-OLED device. The TFT-OLED device is composed of a TFT device 100 and an OLED device 200 that are formed on top of a substrate 10. The substrate 10 in this example is a semiconductor material and, thus, insulating buffer films 11 and 12 of silicon nitride and silicon oxide, respectively, are first formed on the substrate 10. The substrate also may be formed of glass, synthetic resin, or the like also. In which case the buffer films 11 and 12 are not necessary.
The TFT device 100 is formed by first depositing an active layer 102 of a polysilicon film. The active layer 102 is doped on the outer sides to form a source 102S and a drain 102D regions with a channel region 102C in between. A blanket of gate oxide insulation film 20 is then deposited over the active layer 102, covering the active layer 102 as well as the rest of the substrate. A gate electrode 120, typically of a chromium and molybdenum, is then deposited on the gate oxide insulation film 20 positioned directly over the channel region 102C. Next, a blanket of interlayer dielectric (ILD) insulation layer 22 is provided, covering the gate electrode 120 and the rest of the gate oxide insulation film 20. The ILD layer 22 is typically made from silicon nitride or silicon oxide. Via holes 111 and 112 are etched through the two insulation layers, ILD layer 22 and the gate oxide insulation film 20, and down to the drain region 102D and the source region 102S. A drain electrode 110D and a source electrode 110S are formed by filling the respective via holes 111 and 112 with a metal such as aluminum. A first passivation layer 30 is then provided over the entire surface covering the source and drain electrodes 110S, 110D and the necessary structures for the TFT device 100 is now fully formed.
The first passivation layer 30 forms the surface on which the OLED device 200 is formed. To form the OLED device 200, a contact via hole 212 is first etched into the first passivation layer 30 over the source electrode 110S. A transparent electrode constituting the anode 210 of the OLED is deposited on the surface of the first passivation layer 30 including the contact hole 212 so that the anode 210 makes electrical contact with the source electrode 110S through the contact hole 212. The anode 210 is made of a transparent electrically conductive material, typically indium-tin-oxide (ITO). A second passivation layer 32 is provided over the entire surface and an opening is etched into the second passivation layer 32 exposing the anode 210 in the region corresponding to the location of the OLED device 200 being formed. This opening defines a pixel in a display formed by a matrix of these EL display devices and an organic EL emitter layer 215 is deposited over the anode 210 in the opening region. Finally, a cathode 220, typically of aluminum is deposited over the emitter layer 215 completing the OLED structure.
When an appropriate bias potential is created between the anode 210 and the cathode 220, holes and electrons injected from the anode 210 and cathode 220, respectively, are recombined in the emitter layer 215 causing the emitter layer to emit energy as light through the transparent anode 210 and the substrate 10.
In the conventional EL display device, such as the conventional TFT-OLED structure illustrated in FIG. 1, because the electrical contact between the anode 210 of the EL element and the source electrode 110S is made through the contact hole 212, which is small, the contact resistance tends to be high. Typically, the contact holes 212 are about 5-10 um in diameter and the typical contact resistance between aluminum metal and ITO contacting through such a structure is about 50 ohms. This high contact resistance requires more power to drive the EL element. Improving the contact between the anode 210 and the source electrode 110S and lowering the contact resistance would improve the EL display device's power demand.
Another problem with the conventional EL display device structure is that it is difficult to produce an anode layer having a desired flatness. Because the anode 210 of the EL element is deposited over a number of underlying layers of insulation films, it is difficult to control the surface roughness of the anode layer. For a reliable performance of the EL element, it is preferred that the anode 210 meet the flatness (or surface roughness) of Rms<10 Å and Rpv<100 Å.
Thus, an improved EL display device is desired.